Apparatus for measuring ammonia concentration, system for measuring ammonia concentration, system for treating exhaust gas, and method for measuring ammonia concentration

Abstract

A apparatus 70 for measuring ammonia concentration includes an electromotive force acquisition section 75 configured to acquire information about an electromotive force EMF of a mixed potential cell 55 while a detection electrode 51 is exposed to a target gas, an oxygen concentration acquisition section 76 configured to acquire information about oxygen concentration p O2 in the target gas, and a control section 72 . The control section 72 derives ammonia concentration p NH3 in the target gas from the acquired information about the electromotive force EMF, the acquired information about the oxygen concentration p O2 , and the relationship represented by formula (1): EMF=α log a ( p NH3 )−β log b ( p O2 )+ B   (1) where α, β, and B each represent a constant, and a and b each represent any base (provided that a≠1, a>0, b≠1, and b>0).

What is claimed is: 
     
       1. An apparatus for measuring ammonia concentration in a target gas with a sensor element including a mixed potential cell that includes a solid electrolyte body, a detection electrode arranged on the solid electrolyte body, and a reference electrode arranged on the solid electrolyte body, the apparatus comprising:
 an electromotive force acquisition section configured to acquire information about an electromotive force of the mixed potential cell while the detection electrode is exposed to the target gas; 
 an oxygen concentration acquisition section configured to acquire information about oxygen concentration in the target gas; and 
 an ammonia concentration derivation section configured to determine ammonia concentration in the target gas based on the acquired information about the electromotive force, the acquired information about the oxygen concentration, and a relationship represented by formula (1):
   EMF=α log a ( p   NH3 )−β log b ( p   O2 )+ B   (1)
 
 
 
       (where
 EMF: an electromotive force of the mixed potential cell, 
 α,β, and B: constants, 
 a and b: any base (provided that a ≠1, a >0, b ≠1, and b >0), 
 p NH3 : the ammonia concentration in the target gas, 
 p O2 : the oxygen concentration in the target gas). 
 
     
     
       2. A system for measuring ammonia concentration, comprising:
 the apparatus for measuring ammonia concentration according to  claim 1 ; and 
 the sensor element. 
 
     
     
       3. The system for measuring ammonia concentration according to  claim 2 ,
 wherein the detection electrode is composed of a Au—Pt alloy as a main component. 
 
     
     
       4. The system for measuring ammonia concentration according to  claim 3 ,
 wherein the detection electrode has a degree of concentration (=amount of Au present [atom % ]/amount of Pt present [atom % ]) of 0.3 or more, the degree of concentration being measured by at least one of X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). 
 
     
     
       5. The system for measuring ammonia concentration according to  claim 2 ,
 wherein the sensor element includes a heater configured to heat the mixed potential cell to an operating temperature of 450° C. or higher and 650° C. or lower. 
 
     
     
       6. A system for treating an exhaust gas, comprising:
 the system for measuring ammonia concentration according to  claim 2 ; and 
 an exhaust gas path through which an exhaust gas serving as the target gas from an internal combustion engine flows, the sensor element being arranged in the exhaust gas path. 
 
     
     
       7. The system for treating an exhaust gas according to  claim 6 , further comprising:
 one or more oxidation catalysts arranged in the exhaust gas path, 
 wherein the sensor element is arranged on the downstream side of the exhaust gas path in contrast to one of the one or more oxidation catalysts arranged at an upstream end. 
 
     
     
       8. A method for measuring ammonia concentration in a target gas with a sensor element including a mixed potential cell that includes a solid electrolyte body, a detection electrode arranged on the solid electrolyte body, and a reference electrode arranged on the solid electrolyte body, the method comprising:
 an electromotive force acquisition step of acquiring information about an electromotive force of the mixed potential cell while the detection electrode is exposed to the target gas; 
 an oxygen concentration acquisition step of acquiring information about oxygen concentration in the target gas; and 
 a concentration derivation step of determining ammonia concentration in the target gas based on the acquired information about the electromotive force, the acquired information about the oxygen concentration, and a relationship represented by formula (1):
   EMF=α log a ( p   NH3 )−β log b ( p   O2 )+ B   (1)
 
 
 
       (where
 EMF: an electromotive force of the mixed potential cell, 
 α,β, and B: constants, 
 a and b: any base (provided that a ≠1, a >0, b ≠1, and b >0), 
 p NH3 : the ammonia concentration in the target gas, 
 p O2 : the oxygen concentration in the target gas).